Electrochemical processing involves the interaction of electrical and chemical reactions to produce a wide variety of products, separations, and processes. Included among the more significant industries that employ electrochemical processing are the plating industry, the chloro-soda industry, hydrogen-oxygen fuel cells, waste water treatments, and the plethora of industries employing electrochemical membranes, hollow fibers and tubes. Electrochemical processes are used to produce products such as aluminum, deuterium, fluorine, platinum, and sorbitol, to name only a few.
Electrochemical processes are used to convert chemical energy into electrical energy in electrolytic fuel cells, such as batteries. Electrolytic cells usually employ membranes that are permeable to one type of ion but impermeable to the other. Recently, DuPont has developed a perfluorosulfonic membrane known as "Nafion". Nafion membranes have the chemical and thermal stability of Teflon tetrafluoroethylene resins but are very hydrophilic. Unlike Teflon, which is one of the most hydrophobic substances known, Nafion absorbs water rapidly even at room temperature. Their high chemical stabilities and high water absorption rates have made Nafion membranes a unique, long-life separator in electrochemical and chemical processing.
Nafion is a polyperfluorosulfonic acid resin. Membranes made from Nafion resins were a revolutionary development in the field of electrochemistry. The resin and its membranes, especially the composite membrane of polyperfluorosulfonic acid and polyperfluorocarboxylic acid, have been broadly used in all industries where electrochemical membranes are employed.
In the 1950's, chemists began focussing on fluorocarbon polymers having extraordinary chemical stabilities and the mechanical and electrical properties of polytetrafluoroethylene (PTFE) but that were also melt-fabricable like the polyethylenes and polyamides. The rearrangement of hexafluoropropylene epoxide (HFPO) led to the production of perfluoropropionyl fluoride, which is then reacted with more HFPO to produce a dimer which, on heating with sodium carbonate, yields perfluoropropyl vinyl ether (PPVE). PPVE has been copolymerized with tetrafluoroethylene (TFE) to provide a thermoplastic with the chemical stability and mechanical properties closely approaching those of PTFE.
Chemists then discovered the remarkable properties of an ionomer resin which was an acid salt of an ethylene and methacrylic acid copolymer. These ionomers were sold under the trademark "SURLYN". Researchers also began to search for fluorocarbon resin ionomers having acid groups with greater thermal stability than carboxyls in order to find ionomers capable of withstanding the high processing temperatures required for fabricating fluorocarbon plastics.
Then, in the late 1970's, a Bronsted acid of nitrogen was synthesized for the purpose of extending the number of possibilities of xenon-nitrogen bonds. From that research, a class of superacids with considerable promise in electrochemical applications emerged. The superacid developed was a perfluorinated sulfonyl nitrogen acid having the formula (CF.sub.3 SO.sub.2).sub.2 NH.
(CF.sub.3 SO.sub.2).sub.2 NH is more than two orders of magnitude stronger in acidity than nitric acid in acetic acid solvent (dissociation constant of 10.2 vs.7.8). The phase acidity of these compounds show that they far exceed the inherent acidity of other acids such as CF.sub.3 SO.sub.2 OH, FSO.sub.3 H, and HI.
Electrochemical studies have shown that these acids exhibit favorable properties on low surface area and high surface area electrodes employed in practical fuel cells. The compounds greatly improve the output of the typical fuel cell. Electrolytes and fuel cells employing these materials dictate that the fluoropolymers be extremely stable. Presently, Nafion and Dow 560 are the current ionomers of choice in fuel cell applications.
To produce a Nafion polymer, a cyclic sultone is rearranged to a linear form and reacted with HFPO to produce a sulfonyl fluoride, which is then reacted with sodium carbonate to yield a sulfonyl fluoride vinyl ether (PFSEPVE). PFSEPVE is then polymerized with TFE to give a perfluorocarbon sulfonyl fluoride copolymer that can be fabricated into a membrane and other various articles. This polymer has the chemical formula: ##STR3##
The sulfonyl copolymer can be completely saponified with hot caustic to give a sodium salt which can then be converted with an acid to an acid polymer resin form. The salt and free acid forms of the polymer resin are essentially infusible.
Nafion polymers may be fabricated into various forms, including membranes, diaphragms, tubing, laminates, and filaments. These products are shaped as desired by melt fabricating the sulfonyl fluoride copolymer, followed by saponifying and exchanging. In this manner, the Nafion products can be made free of pin holes, which is a necessity for membrane processes.
Nafion membranes have been used in electrolytic fuel cells, electrodialysis processes, including dialysis of brackish water and electrolysis of brine, chrome plating, and other applications. In electrochemical cells, Nafion membranes separate the cell into two compartments and serve as a wettable, ionically conductive, perm selective barrier. In dialysis, Nafion membranes serve as a wettable, perm selective reactor.
Nafion membranes are permeable to positively charged ions (cations) but are impermeable to negatively charged ions (anions). By tailoring the polymer structure and employing special techniques for fabricating and reinforcing the membrane, Nafion membranes combine good selectivity with low resistance, high physical strength and long service life.
The membrane processes employing Nafion membranes are advantageous from an energy standpoint over evaporative and crystallization processes. Processes using these membranes allow separation of dissolved materials from one another or from a solvent with no phase change. Membrane processes do not require the added energy required for vaporization or crystallization. Because energy costs represent a substantial and increasing percentage of the total cost for most separation operations, membrane processes offer significant energy savings. In addition, electrochemical membrane processes offer possible solutions to ecological problems, particularly in the plating industry. Potential pollutants in the plating industry are converted into valuable products by electropurifying and electrooxidzing a process stream to make the stream constituents suitable for reuse.
Nafion membranes, however, suffer from the inability to retain sufficient water to maintain proton conductivity above 80.degree. C. New monomers which form superior polymeric membranes and offer greater flexibility in the design of membranes are currently being sought.
The present invention, which incorporates the ##STR4## acid groups into a perfluorocarhon polymer, can be used to create new and novel polyfluorocarhon electrolytes for improved use in fuel cells and other applications.
Various fluorocarbon compositions having sulfonyl groupings are known in the art. For example, U.S. Pat. No. 3,050,556 to Tiers relates to a mono-chloro-substituted long chain alkanesulfonyl fluoride having the general formula Cl(CHR-CH.sub.2).sub.n SO.sub.2 F where R is an alkyl radical of from 6 to 16 carbons and n is an integer of from 1 to 2. U.S. Pat. No. 3,301,893 to Putnam et al. also relates to various fluorocarhon ethers having the general formula: ##STR5## where R.sub.f is a radical selected from fluorine and perfluoroalkyl radicals having from 1 to 10 carbons, X is a radical selected from fluorine, trifluoromethyl radicals and mixtures thereof, Y is a radical selected from fluorine, amino, hydroxyl and --OMe radicals where Me is a radical selected from ammonium radicals, alkali metals and other monovalent metals, and where n represents a number from 0 to 12. U.S. Pat. No. 3,849,243 to Grot relates to laminates of fluorinated polymers containing pendant side chains having sulfonyl groups where a majority of the sulfonyl group of one surface is in the --(SO.sub.2 NH).sub.m Q form where Q is H, a cation of an alkali metal, or a cation of an alkaline earth metal or combination thereof, and m is the valance of Q, while the sulfonyl groups of the other surface are in the --SO.sub.2 M form wherein M is a halogen atom. One monomer used in making the polymers of the invention disclosed therein has the generic formula CF.sub.2 =CFR.sub.f SO.sub.2 F wherein R.sub.f is a bifunctional perfluorinated radical comprising 1 to 8 carbons.
Other patents and applications, such as U.S. Pat. Nos. 4,578,512 to Ezell et al., 4,337,211 to Ezell et al., 4,734,474 to Hamada et al., 4,554,112 to Ezell et al., 4,474,400 to Krespan, German Patent No. 1959142 to Abitz et al. and EPO Publication Nos. 0062323 of Darling and 0062324 of Krespan et al., show various perfluorinated monomer compounds and polymers resulting therefrom.
Although various perfluorinated monomers containing sulfonyl groups are known, the particular features of the present invention are absent from the art. The prior art is generally deficient in affording a non-oxy perfluorinated superacid monomer for producing an ionomer membrane having the characteristics and flexibility of the presently claimed invention. The present invention overcomes the shortcomings of the prior art in that the monomers disclosed herein and polymers made therefrom result in higher chemical stabilities and physical flexibilities than the Nafion polymers while providing all the desired characteristics of a Nafion polymer.